Most hormones, neurotransmitters and pharmaceutical drugs bind to G protein coupled receptors (GPCRs) - cell surface receptors that activate intracellular heterotrimeric G proteins (G???) through a long- standing and well-characterized mechanism. Numerous diseases have been associated with mis-regulation or mutation that affects the proper reception and transduction through these signaling pathways. Thus, G protein signaling impacts most areas of human health, and understanding the mechanisms that regulate G protein signaling is expected to improve the therapies for such diseases. Emerging evidence from mammalian model systems have identified many diverse, yet poorly understood, signaling roles for G?? subunits. We have discovered a new mechanism of G protein signaling regulation mediated by phosphorylation of G? subunits in their unstructured N-terminal tails (G?-Nt). G?-Nt phosphorylation has been found on many different G? subunits including those from yeast, mice and humans but its role in signaling has never been determined. Our long-term goal is to understand the general mechanism, function and dynamics of G?-Nt phosphorylation and its evolved role as a regulator of G protein signaling in eukaryotic organisms including humans. We will conduct a complete and rigorous analysis of the causes, effects and mechanisms underlying the phosphorylation of G? N-terminal tails. Since this is the first study on a new G protein regulatory mechanism we will exploit the yeast model, which harbors a single canonical G protein signaling system with all G?-Nt regulatory elements intact. In Aim 1 we will determine the necessary conditions for G?-Nt phosphorylation and its role in G protein signaling, including the activation of mitogen-activated protein kinases (MAPKs) and chemotropism. The hypothesis is that phosphorylation of G?-Nt is a negative feedback response to cellular stress that antagonizes sustained G protein-dependent pathway activation. In Aim 2 we will identify kinases necessary and sufficient for G?-Nt phosphorylation. We will test the hypothesis that multiple stress-activated kinases phosphorylate the N-terminal tail of G? subunits to attenuate G protein signaling in an adaptive response to cellular stress. The successful completion of this aim will provide foundational evidence of the kinases and pathways leading to G?-mediated stress adaptation. In Aim 3 we will determine how G?-Nt phosphorylation regulates the function of G protein subunits and subunit/effector interactions. We will specifically test the hypothesis that G?-Nt phosphorylation alters the protein stability and/or effector interactions of G?? subunits, thereby antagonizing normal G protein signaling. Completion of the proposed experiments will exhaustively define the parameters governing the phosphorylation of a canonical G? N-terminal tail - the conditions and kinases involved; the effects on the G proteins themselves and on signal transduction outputs, including both MAPK activation and chemotactic/tropic proficiency. Evidence from these studies is expected to provide a foundation for further work into the nature of this newly discovered regulator of G protein signaling.